Several prosthetic valves are known. See, for example, U.S. Pat. No. 5,411,552 (Andersen et al.), entitled VALVE PROSTHESIS FOR IMPLANTATION IN THE BODY AND CATHETER FOR IMPLANTING SUCH VALVE PROSTHESIS, which discloses a valve prosthesis comprising a stent made from an expandable cylinder-shaped thread structure comprising several spaced apices. See, also, U.S. Pat. No. 6,168,614 (Andersen et al.), entitled VALVE PROSTHESIS FOR IMPLANTATION IN THE BODY, U.S. Pat. No. 5,840,081 (Andersen et al.), entitled SYSTEM AND METHOD FOR IMPLANTING CARDIAC VALVES, and PCT Application No. PCT/EP97/07337 (Letac, Cribier et al.), published as WO 98/29057, entitled VALVE PROSTHESIS FOR IMPLANTATION IN BODY CHANNELS, all of which are incorporated herein by reference.
In the development of stented valves, a highly desirable, and often preferred design utilizes a cylindrical stent platform of either balloon expandable or self-expanding metal designs. Usually these stents follow the cellular designs which tend to have higher radial strength and less foreshortening than wire-wound platforms.
Such cylindrical stents offer a stable and reproducible expansion platform for attaching valves and may be manufactured from a variety of biocompatible metals including stainless steels, titanium alloys, platinum-iridium, nickel-titanium alloys, chromium alloys, or tantalum.
Polymeric, bovine venous, pericardial, and porcine valve constructs have lead the early development efforts of stent-valve designs. All of the early designs have utilized either bicuspid or tricuspid valve designs.
One of the key factors that determines the long term functionality of stented valves is the retrograde flow characteristics. The retrograde flow characteristics, along with the stiffness characteristics of the valve material, will determine leakage and closing pressure requirements. The retrograde flow characteristics are most important in low flow/low pressure systems where the valve leaflets may thrombose in the presence of poor retrograde laminar flow.
Stented valves are passive devices. The valves function as a result of changes in pressure and flow. An aortic stented valve opens passively when the pressure in the left ventricle exceeds the pressure in the aorta (plus any resistance required to open the valve). The valve closes when the pressure in the left ventricle is less than the pressure in the aorta. However, the flow characteristics are critical to effect the closing of the aortic valve, otherwise regurgitation will ensue.
Laminar flow is the normal condition found in most of the circulatory system. It is characterized by concentric layers of blood moving in parallel down the length of the blood vessel. The highest velocity is found in the middle of the blood vessel while the lowest is found along the wall. The flow is parabolic in a long straight vessel under steady flow conditions.
Non-laminar, or turbulent, flow is useful to the circulatory system. For example, the aortic valve opens into the sinus of Valsalva at the inferior aspect of the ascending aorta. This sinus has two key functions: First, it maximizes the flow characteristics so that the aortic valve closes during diastole. And second, it optimizes coronary sinus flow and perfusion.
Laminar flow makes the retrograde flow characteristics of valves mounted in cylindrical stents problematic as the flow along the wall is least, which is central to the closing of a valve. Such laminar flow with its attendant drawbacks is a characteristic of known stented valves. There is a need to have stented valves where the retrograde flow characteristics will be non-laminar, which will be advantageous with regard to valve closing.